1. Field of the Invention
The present invention relates to an injection-molding machine and, particularly, to a system for controlling the nozzle touch force of the injection-molding machine.
2. Description of the Related Art
In a nozzle touch mechanism which moves an injection unit as a whole to bring it into contact with a metal mold, the injection unit mounted on a base so as to move back and forth is driven by a nozzle touch force generating device to bring the nozzle of the injection unit into contact with the metal mold. A known nozzle touch force generating device utilizes a hydraulic cylinder or a spring force expanded by the force of a motor to hold the nozzle in contact with to the metal mold (see Japanese Examined Patent Publication (Kokoku) No. 6-17038)
There is further known a nozzle touch mechanism which pushes the nozzle onto the metal mold with a predetermined pressure by using a hydraulic pressure. The nozzle touch force has heretofore been controlled in a manner as described above, i.e., the output of the nozzle touch force generating device is maintained constant to hold the nozzle so as to be pushed onto the metal mold.
There is further known a control method in which an injection unit is provided with a distortion sensor to set the nozzle touch force easily and at any value, and a servo motor is controlled relying on a feedback signal from the distortion sensor to advance the injection unit thereby to obtain a desired nozzle touch force (see Japanese Examined Patent Publication (Kokoku) No. 7-106580).
In order to cancel the reaction force generated by the force of inertia of the moving member of the injection unit at the time of acceleration or deceleration of injection, there is also known an invention according to which moving members are provided to move in directions opposite to each other, and the force of inertia is excluded so that the members of the injection-molding machine will not be broken or the control characteristics deteriorated (see Japanese Unexamined Patent Publication (Kokai) No. 2001-124169).
There is further known an invention according to which the reaction force in the axial direction acting on a screw is detected by using a load cell provided downstream of the screw, the force of inertia of the moving part such as the screw that is generated at the time of acceleration or deceleration of injection is found to correct the reaction force, in order to correctly find the pressure exerted on the molten resin (see Japanese Unexamined Patent Publication (Kokai) No. 2003-191285).
To prevent leakage of the molten resin from a position where the nozzle and the metal mold are in contact with each other, it is desirable to maintain constant the nozzle touch force in the nozzle touch mechanism. At the time of acceleration of injection, however, the nozzle touch force for pushing the nozzle touch surface drops, being affected by the force of inertia of the moving member such as the injection screw or the pusher plate of the injection unit. FIGS. 9a and 9b are diagrams illustrating the nozzle touch force in a conventional nozzle touch force generating device by utilizing a spring. FIG. 9a illustrates the injection speed of when the injection screw advances in the injection unit to inject the molten resin in the heating cylinder into a metal mold, and FIG. 9b illustrates the nozzle touch force at the time of injection.
When the injection operation starts in a state where a predetermined nozzle touch force fs is generated due to the compression of the spring, there occurs a drop in the nozzle touch force for pushing the nozzle touch surface at the time of acceleration of injection due to the force of inertia of the moving members such as the injection screw and the pusher plate in the injection unit. At the time of deceleration of injection, on the other hand, the nozzle touch force increases due to the force of inertia of the moving part of the injection unit. FIG. 9b illustrates an increase and a decrease of the nozzle touch force by f1 at the time of acceleration and deceleration of injection.
When the nozzle touch force becomes smaller than a force produced by the resin pressure, the injection unit is pushed back, permitting the resin to leak. So far, therefore, the nozzle touch surface had been pushed with a sufficiently large nozzle touch force so that there was no leakage of resin despite the nozzle touch force having dropped being affected by the acceleration of injection. When pushed with a large nozzle touch force, however, the metal mold deflects and/or a parallel relationship of the mold is not maintained, giving rise to problems with regard to a drop in the quality of the molded article and a decrease in the life of the metal mold. As shown in FIG. 9b, also, the nozzle touch force increases due to the force of inertia at the time of deceleration in the injection speed, causing a further decrease in the life of the metal mold.